A video which appears to show an Islamic State-linked group in Algeria behead a French hostage has appeared on the internet.
Herve Gourdel, a 55-year-old hiker, was captured in Algeria on Sunday.
France has been involved in air strikes against IS militants in Iraq.
A 55-year-old Frenchman kidnapped by a splinter group from al Qaida's North African branch will be killed unless his homeland halts air strikes against the Islamic State group, his captors say.
In a video that appeared on social media, a masked member of a group calling itself Jund al-Khilafah, or Soldiers of the Caliphate, addressed the threat to President Francois Hollande and said the hostage would be killed unless the air strikes were halted within 24 hours.
The group said it was answering a call by Islamic State (IS) spokesman Abu Muhammad al-Adnani to attack Americans and Europeans.
France's Foreign Ministry said the video was authentic and identified the hostage as mountain guide Herve Gourdel, 55.
Tributes have poured in for David Morgan, the British teacher who is understood to have been killed when the Air Algerie flight crashed in Mali.
PE teacher Andy Vasily posted tweeted: "RIP Dave Morgan. Truly tragic. Prayers to his family and other victims."
Former @nischina teacher killed in Algerian plane crash. RIP Dave Morgan. Truly tragic. Prayers to his family and other victims.
He added that Mr Morgan had previously worked at Nanjing International School in eastern China.
Others including Hanri de Swardt from South Africa, paid their respects on Facebook.
De Swardt, a fellow teacher, wrote: "Rest in peace Captain Dave Masher Morgan. Thanks for all the support and help in Lusaka. You will always be in my thoughts. God bless."
She added: "Remember the nice evening at Brentwood with all our last food before leaving Zambia."
Another teacher Annabelle Mambwe from the Luanda International School, in Angola, wrote: "Just lost a teacher colleague in the Algiers disaster. Dave Morgan!! He was Denise's Maths teacher...such an easy-go-fella! These planes falling from the sky are getting to me!"
A British man killed when an Air Algerie flight crashed in northern Mali is understood to be David Morgan, ITV News has learnt.
Prime Minister David Cameron earlier said he was "deeply saddened" by the death of the British national.
At least 118 people died in the disaster, with nearly half of those aboard the flight believed to be French nationals.
French government officials have revised the death toll of the Air Algerie plane crash to 118, without explaining the reason for the change.
They also raised the number of French killed from 51 to 54.
Aerial footage of the site where Flight AH5017 crashed was also released by regional military authorities this evening.
Prime Minister David Cameron said he was "deeply saddened" by the death of a British man on the Air Algerie flight AH5107.
"Thoughts very much with friends and family," he added.
Mr Cameron has also written to Mr Hollande "to send sincere condolences for the loss of so many French citizens", Downing Street said.
A strong smell of aircraft fuel and the fact that debris was scattered over a small area suggests the Air Algerie plane crash was linked to weather or a technical problem, French Transport Minister Frederic Cuvillier has said.
Aviation officials lost contact of flight AH5017 at around 0155 GMT on Thursday, less than an hour after taking off for Algeria, following a request by the pilot to change course due to bad weather.
One of the two black boxes from the crashed Air Algerie flight has been sent to the Malian city of Gao for formal identification before being repatriated, French Foreign Minister Laurent Fabius has said.
A French contingent of troops is based in the city, which is a government-controlled town.
The Air Algerie AH5107 flight crashed in the northern Mali desert killing all 116 people on board.
The Gossi region where the accident occurred, near the Burkina Faso border, is 100 miles south of Gao.
French president Francois Hollande has said all avenues are being discussed after flight AH5017 crashed in the northern Mali desert, killing 116 people.
Nearly half of the passengers aboard the flight were French, many heading to Europe.
Speaking to reporters, Mr Hollande said that France will spare no efforts to find out what had happened.
"There are hypotheses, notably weather-related, but we don't rule out anything because we want to know what happened," the French president said.
Interior minister Bernard Cazeneuve added: "Terrorist groups are in the zone. ... We know these groups are hostile to Western interests."
A British man died onboard Air Algerie flight AH 5107, which crashed in the northern Malian desert killing least 116 people, the Foreign Office have said today.
It is with deep regret that the Foreign and Commonwealth Office confirms the death of a British man onboard Air Algerie flight AH 5107.
We are providing consular support to his family at this tragic time, and we ask that the media respect the privacy of those grieving.